Truck and bus suspensions utilize at least one torque rod to secure the drive axle to the vehicle's frame. The securing of the drive axle to the vehicle's frame by the torque rod maintains the drive axle's alignment to the vehicle's frame, it maintains the proper suspension geometry for the vehicle, and it allows free suspension movements in jounce and rebound for all terrain, road and driving conditions. Because of the wide range of dynamic operating conditions for these vehicles, especially heavy duty trucks, the severe impact loads to the suspension system combined with the road induced vibrations on the suspension system lead to a deleterious effect on the individual suspension components including the torque rods as well as having a negative impact on the operator's physical fatigue condition. These severe dynamic conditions can accelerate wear of the torque rods of the suspension system leading to premature failures of these torque rods.
The purpose of torque rods on large vehicles is to stabilize the axle. They prevent the axle from rotating about its axis; they prevent the axle for moving fore and aft during braking and acceleration; and they prevent axle yaw. While there are a variety of suspension designs, one of two approaches are generally used to stabilize the axle. The first approach uses straight rods with pivotal joints at either end. Two of these straight rods are mounted fore and aft on the vehicle; where one end is mounted to the axle and the other end is mounted to the frame. A third straight rod is similarly mounted laterally in the vehicle, generally perpendicular to the other two. The second approach is a V-configuration torque rod assembly. This type of torque rod has a pivotal joint mounted at the apex of the V as well as at the ends of the legs. The apex is mounted to the axle, and the legs are mounted to the frame. The V-configuration controls both fore-aft movement as well as lateral movement. The major advantage of the V-configuration rod assembly is axle stability.
A typical prior art single or V-configuration rod is comprised of two or three pivotal joint eyelet forgings rigidly connected with tubes to provide the mechanical integrity. The eyelets and tubes form a natural path for shock and vibration energy to transfer from the suspension system into the frame, the cab and other areas of the sprung mass of the vehicle. In order to intercept this path, attempts have been made to incorporate an isolation function into the pivotal joint design. This isolation function thus makes the pivotal joint a critical multi-functional component for the torque rod assembly as well as the suspension system as a whole.
Current pivotal joint designs are based on at least one of two product principles. The first is that the designs incorporate flexible elastomeric bushings and the second is that the designs incorporate metal on metal or metal/plastic components. Both of these designs have their individual advantages but because of their performance capability limitations, neither one encompasses the needed combination of noise, vibration and harshness (NVH) isolation capability; torsional or oscillatory freedom; and lateral spring rate control. Each of these three characteristics are essential elements for the optimum isolation while maintaining the necessary vehicle handling and stability. All three characteristics have a significant impact on NVH and handling properties thus leading to a significant affect on the operator's physical condition. The pivotal joint designs that incorporate a flexible elastomeric bushing are known for their good isolation capability but their inherent flexibility may compromise the stability that can be provided by a rigid joint at the apex of a V-configuration rod. In addition, the limited torsional oscillatory angle capability of the flexible elastomeric bushing and the inherent zero point of the torsional spring complicates the installation of torque rods having this type of pivotal joint. The pivotal joint designs that incorporate rigid metal or metal/plastic ball joint pivots are based on the sliding bearing product principle. Although the sliding bearing principle provides suspension freedom in the vertical plane and requires a relatively simple installation process, the bearing's rigidity limits its isolation capability. Consequently, it acts as a conduit for road induced impact and vibrations into the frame and eventually into the cab. Other disadvantages of the rigid metal or metal/plastic designs is that they consist of numerous costly semi precision bearing components requiring expensive boot seals that are vulnerable to wear, tear, exterior cuts and the like. In addition, the sliding bearing designs require periodic maintenance and lubrication.
In the V-configuration (three pivot joint designs), the most critical component is the apex pivotal joint which is normally positioned on the drive axle. This apex pivotal joint design is critical because it is subjected to double the load of the individual arm or frame connection pivots; it is subjected to greater conical displacement than the frame connection pivots; it is the closest to the source of road induced impacts, tire noise and vibration in general; and its fore and aft and lateral mode spring rate ratio affects the pivotal joint's isolation effectiveness and vehicle stability.
The continued development of pivotal joints for suspension components, and especially the apex pivotal joint, has been directed towards designing pivotal joints which are able to maximize their isolation capabilities while simultaneously maximizing the stability provided by the pivotal joint.